Silicon wafer and production method thereof and evaluation method for silicon wafer

ABSTRACT

A silicon wafer obtained by slicing a silicon single crystal ingot grown by the Czochralski method with or without nitrogen doping, wherein the silicon wafer has an NV-region, an NV-region containing an OSF ring region or an OSF ring region for its entire plane and has an interstitial oxygen concentration of 14 ppma or less, and a method for producing it, as well as a method for evaluating defect regions of a silicon wafer. Thus, there are provided a silicon wafer that stably provides oxygen precipitation regardless of position in crystal or device production process, and a method for producing it. Further, defect regions of a silicon wafer of which pulling conditions are unknown and thus of which defect regions are also unknown can be evaluated.

TECHNICAL FIELD

The present invention relates to a silicon wafer in which oxygenprecipitation is stably obtained regardless of device production processand position in crystal and a method for producing it, as well as amethod for evaluating defect regions of a silicon wafer of which pullingconditions are unknown.

BACKGROUND ART

In recent years, in connection with the use of finer devicesaccompanying the use of higher integration degree of semiconductorcircuits such as DRAM, demand for quality of silicon single crystalsproduced by the Czochralski method (it may be also abbreviated as “CZmethod” hereinafter) from which substrates therefor are produced isbecoming higher. In particular, since there are defect called grown-indefects such as FPD, LSTD and COP and they degrade devicecharacteristics, reduction of these defects is considered important.

Prior to explanation of those defects, there will be given first generalknowledge of factors determining densities of defects introduced intosilicon single crystals, a void type point defect called vacancy (alsoabbreviated as V hereinafter), and an interstitial type silicon pointdefect called interstitial silicon (interstitial-Si, also abbreviated asI hereinafter).

A V-region in a silicon single crystal means a region containing manyvacancies, i.e., depressions, holes and so forth generated due toshortage of silicon atoms, and an I-region means a region containingmany dislocations and aggregations of excessive silicon atoms generateddue to excessive amount of silicon atoms. Between the V-region and theI-region, there should be a neutral region (also abbreviated as N-regionhereinafter) with no (or little) shortage or no (or little) surplus ofthe atoms. Further, it has become clear that the aforementioned grown-indefects (FPD, LSTD, COP etc.) should be generated strictly only withsupersaturated V or I, and they would not be present as defects eventhough there is little unevenness of atoms so long as V or I is notsaturated.

It is known that densities of these two kinds of point defects aredetermined by the relationship between the crystal pulling rate (growingrate), and the temperature gradient G in the vicinity of thesolid-liquid interface in the crystal in the CZ method. It has also beenconfirmed that defects distributed in a ring shape called OSF (OxidationInduced Stacking Fault) are present in the N-region between the V-regionand the I-region. Since OSFs are generated in a shape of concentriccircle observed in a wafer surface when the wafer is sliced from asingle crystal, there is used a term of OSF ring.

Those defects generated during the crystal growth are classified asfollows. For example, when the growth rate is relatively high, i.e.,around 0.6 mm/min or higher, grown-in defects considered to beoriginated from voids, i.e., aggregations of vacancy-type point defects,such as FPD, LSTD and COP, are distributed over the entire cross-sectionof the crystal along the radial direction at a high density, and aregion containing such defects is called V-rich region (region in whichsupersaturated vacancies form void defects). When the growth rate is 0.6mm/min or lower, with the decrease of the growth rate, theaforementioned OSF ring is initially generated at the circumferentialpart of the crystal, and L/D (large dislocations, abbreviation ofinterstitial dislocation loops, which include LSEPD, LFPD and so forth),which are considered to be originated from dislocation loops, arepresent outside the ring at a low density. A region containing suchdefects is called I-rich region (region in which supersaturatedinterstitial silicons form dislocation loop defects). When the growthrate is further lowered to around 0.4 mm/min or lower, the OSF ringshrinks and disappears at the center of wafer, and thus the entire planebecomes the I-rich region.

Recently, there has been discovered a region called N-region between theV-rich region and the I-rich region, and outside the OSF ring, in whichneither of the void-originated FPD, LSTD and COP, the dislocationloop-originated LSEPD and LFPD and OSF are present. This region existsoutside the OSF ring, and shows substantially no oxygen precipitationwhen it is subjected to a heat treatment for oxygen precipitation andexamined by X-ray analysis or the like as for the precipitationcontrast. This region is present at rather I-rich side, and theinterstitial silicon point defects are not so rich as to form LSEPD andLFPD.

Presence of the N-region was also confirmed inside the OSF ring, inwhich neither of void-originated defects, dislocation loop-originateddefects and OSFs were present.

Because these N-regions are formed obliquely with respect to the growingaxis when the growth rate is lowered in a conventional growing method,it exists as only a part of the wafer plane.

As for this N-region, according to the Voronkov's theory (V. V.Voronkov, Journal of Crystal Growth, 59 (1982) 625-643), it was proposedthat a parameter of F/G, which is a ratio of the pulling rate (F) andthe crystal solid-liquid interface temperature gradient (G) along thegrowing axis, determined the total density of the point defects. In viewof this, because the pulling rate should be constant in a plane, forexample, a crystal having a V-rich region at the center, I-rich regionat the periphery, and N-region between them is inevitably obtained at acertain pulling rate due to distribution of G in the plane.

Therefore, improvement of such distribution of G has recently beenattempted, and it has become possible to produce a crystal having theN-region spreading over an entire transverse plane of the crystal, whichregion could previously exist only obliquely, for example, at a certainpulling rate when the crystal is pulled with a gradually decreasingpulling rate F. Further, such an N-region spreading over an entiretransverse plane can be made larger to some extent along thelongitudinal direction of the crystal by pulling the crystal at apulling rate maintained at the value at which the N-region transverselyspreads. Furthermore, it has also become possible to make the N-regionspreading over the entire transverse plane somewhat larger along thegrowing direction by controlling the pulling rate considering thevariation of G with the crystal growth to compensate it, so that the F/Gshould strictly be maintained constant.

As further classification of the N-region, it is known that there areNV-region (region in which there are many vacancies, but void defectsare not detected), which is present outside the OSF ring and adjacent toit, and NI-region (region in which there are many interstitial siliconsbut dislocation loop defects are not detected), which is adjacent to theI-rich region.

Furthermore, in a silicon substrate produced by the CZ method, controlof oxygen precipitation is becoming increasingly important in view ofinternal gettering effect against heavy metal impurities in addition tothe importance of the reduction of such grown-in defects. However, sincethe oxygen precipitation strongly depends on the heat treatmentconditions, it is a very difficult problem to obtain suitable oxygenprecipitation in the device production process, which may be differentfor every user. Furthermore, wafers are subjected to a heat treatmentnot only in the device production step, but also a heat treatment in thecrystal pulling step, in which the temperature is changed from themelting point to room temperature (thermal history of crystal).Therefore, in an as-grown crystal, there already exist oxygenprecipitation nuclei formed during the thermal history of the crystal(grown-in precipitation nuclei). Such presence of grown-in precipitationnuclei makes the control of oxygen precipitation still difficult.

The oxygen precipitation process in the device production process can beclassified into two kinds of processes. One is a process in whichgrown-in precipitation nuclei that remained after the initial heattreatment of the device production step grow, and the other one is aprocess in which nuclei generated during the device production stepgrow. In the latter case, since the oxygen precipitation stronglydepends on oxygen concentration, it can be controlled by controlling theoxygen concentration. On the other hand, in the former case, thermalstability of grown-in precipitation nuclei (i.e., at how much degree ofdensity they can remain at the temperature of the initial stage of theprocess) constitutes an important point.

For example, even if the grown-in precipitation nuclei exist at a highdensity, if they have a small size, they become thermally unstable anddisappear during the initial heat treatment of the device productionprocess. Thus, oxygen precipitation cannot be secured. The problem inthis case is that, since the thermal stability of grown-in precipitationnuclei strongly depends on the thermal history of crystal, oxygenprecipitation behavior may markedly vary in the device production stepdepending on the crystal pulling conditions or position in the crystalalong the crystal axis even for wafers having the same initial oxygenconcentration. Therefore, in order to control the oxygen precipitationin the device production step, it is important to control not only theoxygen concentration but also the thermal stability of grown-inprecipitation nuclei by controlling the thermal history of crystal.

Although development of techniques for reducing the aforementionedgrown-in defects is currently proceeding, the thermal history of lowdefect crystals produced by such techniques is controlled in order toreduce grown-in defects. It is considered that this also changes thethermal stability of grown-in precipitation nuclei. However, it is notknown at all how much degree it is changed.

Therefore, it is expected that the oxygen precipitation behavior in suchlow defect crystals may significantly vary in the device productionstep, and it results in reduction of device yield.

Further, since no method for judging from which defect region a wafer isproduced has been established as for a wafer of which defect regions areunknown, it is difficult to predict oxygen precipitation behavior in thewafer during the device production step.

DISCLOSURE OF THE INVENTION

Therefore, the present invention was accomplished in view of theaforementioned problems, and its object is to provide a silicon waferthat stably provides oxygen precipitation regardless of position incrystal or device production process, and a method for producing it.Another object of the present invention is to provide a method forevaluating defect regions of a silicon wafer of which pulling conditionsare unknown and thus of which defect regions are also unknown.

The present invention was accomplished in order to achieve theaforementioned objects, and provides a silicon wafer having anNV-region, an NV-region containing an OSF ring region or an OSF ringregion for the entire plane of the silicon wafer and having aninterstitial oxygen concentration of 14 ppma (Japan Electronic IndustryDevelopment Association (JEIDA) Standard) or less.

If an entire plane of silicon wafer consists of a NV-region, OSF ringregion or both of them as described above, there would be an appropriateamount of thermally stable large grown-in precipitation nuclei,therefore fluctuation of oxygen precipitation becomes little even if thedevice process may be different, and BMDs (oxide precipitates calledBulk Micro Defects) can be stably obtained. Further, if an interstitialoxygen concentration is 14 ppma or less, density of small grown-inprecipitation nuclei becomes low, and therefore the wafer can be asilicon wafer showing reduced fluctuation of oxide precipitatesdepending on the position in crystal.

The present invention also provides a silicon wafer obtained by slicinga silicon single crystal ingot grown by the Czochralski method withnitrogen doping, wherein the silicon wafer has an NV-region, anNV-region containing an OSF ring region or an OSF ring region for itsentire plane.

If a silicon wafer is doped with nitrogen and has a NV-region, OSF ringregion or both of them for the entire plane as described above,thermally stable large grown-in precipitation nuclei will be obtained ata high density, and therefore it becomes a silicon wafer providingsufficient gettering effect in the device production process.

In this case, nitrogen concentration doped in the silicon wafer is1×10¹⁰ to 5×10¹⁵ number/cm³.

That is, the above range was defined because at least a nitrogenconcentration of 1×10¹⁰ number/cm³ or more is necessary in order toobtain BMDs at an extremely high density by the effect of nitrogendoping, and a concentration of more than 5×10¹⁵ number/cm³ may inhibitsingle crystallization during the pulling of a single crystal ingot bythe CZ method.

Further, also in the case of a wafer doped with nitrogen, if theinterstitial oxygen concentration is 14 ppma or less, the density ofsmall grown-in precipitation nuclei become low, and hence thefluctuation of oxide precipitates depending on the position in crystalcan be reduced.

The present invention also provides a method for producing a siliconwafer, wherein, when a silicon single crystal is grown by theCzochralski method, the crystal is pulled with such conditions aspresent in an NV region or an OSF ring region in a defect distributionchart showing defect distribution which is plotted with D [mm] in thehorizontal axis and F/G [mm²/° C.·min] in the vertical axis, wherein Drepresents a distance between center of the crystal and periphery of thecrystal, F [mm/min] represents a pulling rate and G [° C./mm] representsan average temperature gradient in the crystal along the crystal pullingaxis direction in the temperature range of from the melting point ofsilicon to 1400° C., so that interstitial oxygen concentration shouldbecome 14 ppma or less.

If a crystal is pulled while the pulling rate F of the crystal and theaverage temperature gradient G in the crystal along the crystal pullingaxis direction in the temperature range of from the melting point ofsilicon to 1400° C. are controlled so that the conditions should bepresent in a region defined by a boundary between a V-rich region and anNV-region and a boundary between an NV-region and an NI-rich region inthe defect distribution chart shown in FIG. 8, which was obtainedthrough analysis of the results of experiments and researches, asdescribed above, a silicon wafer obtained by slicing the grown singlecrystal ingot can have an NV-region, NV-region containing an OSF ringregion or an OSF ring region for its entire plane, and at the same time,the crystal can be pulled so that an interstitial oxygen concentrationshould become 14 ppma or less.

In such a region, there would be an appropriate amount of thermallystable large grown-in precipitation nuclei. Therefore, fluctuation ofoxygen precipitation becomes little even if the device process may bedifferent, and BMDs can be stably obtained. Further, because theinterstitial oxygen concentration is 14 ppma or less, density of smallgrown-in precipitation nuclei becomes low, and therefore the fluctuationof oxide precipitates depending on the position in crystal can bereduced.

The present invention also provides a method for producing a siliconwafer, wherein, when a silicon single crystal is grown by theCzochralski method, the crystal is pulled with such conditions aspresent in an NV region or an OSF ring region in a defect distributionchart showing defect distribution which is plotted with D [mm] in thehorizontal axis and F/G [mm²/° C.·min] in the vertical axis, wherein Drepresents a distance between center of the crystal and periphery of thecrystal, F [mm/min] represents a pulling rate and G [° C./mm] representsan average temperature gradient in the crystal along the crystal pullingaxis direction in the temperature range of from the melting point ofsilicon to 1400° C., and with nitrogen doping.

If a crystal is pulled with such conditions as described above, asilicon wafer obtained by slicing the grown single crystal ingot can bedoped with nitrogen and have an NV-region, an NV-region containing anOSF ring region or an OSF ring region for its entire plane.

If a wafer is doped with nitrogen and has an NV-region, OSF ring regionor both of them for its entire plane as described above, thermallystable large grown-in precipitation nuclei can be obtained at a highdensity, and therefore a wafer that can provide sufficient getteringeffect in the device production process can be produced.

In this case, nitrogen concentration to be doped can be 1×10¹⁰ to 5×10¹⁵number/cm³.

Further, also in this case, when a crystal is grown by the CZ method,the crystal can be pulled so that the interstitial oxygen concentrationshould become 14 ppma or less.

In order to obtain BMDs in an extremely high density by the effect ofnitrogen doping as described above, a nitrogen concentration of 1×10¹⁰number/cm³ or more is necessary. But if the concentration exceeds 5×10¹⁵number/cm³, the single crystallization may be inhibited during thepulling of a single crystal ingot by the CZ method. Therefore, aconcentration of 5×10¹⁵ number/cm³ or less is preferred.

Further, even when nitrogen is doped, if the interstitial oxygenconcentration is 14 ppma or less, the density of small grown-inprecipitation nuclei becomes low. Therefore, the fluctuation of oxideprecipitates depending on the position in crystal can be reduced.

The present invention further provides a method for evaluating defectregions of a silicon wafer produced by the CZ method, wherein a defectregion of a silicon wafer to be evaluated is evaluated by comparing atleast two of oxide precipitate densities measured by the followingsteps:

(1) a wafer to be evaluated is divided into two or more pieces (A, B, .. . ),

(2) Wafer piece A among the divided pieces is loaded into a heattreatment furnace maintained at a temperature of T1 [° C.] selected froma temperature range of 600-900° C.,

(3) the temperature is increased from T1 [° C.] to a temperature of1000° C. or higher, T2 [° C.], at a temperature increasing rate of t [°C./min] (provided that t is 3° C./min or less), and the temperature ismaintained until oxide precipitates in Wafer piece A grow to have adetectable size,

(4) Wafer piece A is unloaded from the heat treatment furnace, and oxideprecipitates density in the wafer piece is measured,

(5) another wafer piece among the divided wafer pieces, Wafer piece B,is loaded into a heat treatment furnace maintained at a temperature ofT3 [°0 C.] selected from a temperature range of 800-1100° C. (providedthat T1<T3<T2),

(6) the temperature is increased from T3 [° C.] to the temperature of T2[° C.] at the temperature increasing rate of t [° C./min], and thetemperature is maintained until oxide precipitates in Wafer piece B growto have a detectable size, and

(7) Wafer piece B is unloaded from the heat treatment furnace, and oxideprecipitates density in the wafer piece is measured.

As for a wafer of which defect regions are unknown, no method forjudging from which defect region the wafer is produced has hitherto beenestablished. Therefore, it has been difficult to predict oxygenprecipitation behavior of the wafer in the device production step.However, by the aforementioned method for evaluating defect regions,defect regions of wafer of which pulling conditions are unknown and thusof which defect regions are also unknown can be evaluated, and at thesame time, it becomes possible to predict oxygen precipitation behaviorof the wafer in the device production step.

As described above, according to the present invention, stable oxygenprecipitation can be obtained regardless of the position in crystal orthe device production process, and therefore a wafer showing littlefluctuation of oxide precipitate density and stable gettering abilitycan be obtained. Furthermore, by using the evaluation method of thepresent invention, defect regions of wafer of which pulling conditionsare unknown and thus of which defect regions are also unknown can bejudged relatively easily.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 shows graphs representing relationship between heat treatmentstarting temperature and BMD density for (a) region inside the OSF ring,(b) OSF ring region, (c) NV-region, and (d) NI-region and I-rich region.

FIG. 2 shows graphs representing oxygen concentration dependency of BMDdensity:

(a) density difference of BMD density for 700° C. and BMD density for800° C., i.e., density distribution for only extremely smallprecipitation nuclei according to position in crystal,

(b) difference of BMD densities for 800° C. and 900° C., and densitydistribution according to position in crystal,

(c) difference of BMD densities for 900° C. and 1000° C., and densitydistribution according to position in crystal, and

(d) BMD density distribution according to position in crystal for 1000°C. or higher.

FIG. 3 shows graphs representing the results for oxygen concentrationdependency of BMD density considering influences of defect region:

(a) difference of BMD densities for 700° C. and 800° C., and BMD densitydistribution according to defect region,

(b) difference of BMD densities for 800° C. and 900° C., and BMD densitydistribution according to defect region,

(c) difference of BMD densities for 900° C. and 1000° C., and BMDdensity distribution according to defect region, and

(d) BMD density distribution according to defect region for 1000° C. orhigher.

FIGS. 4(a) to (f) show graphs representing distribution of BMD densityin plane for high oxygen concentration wafers.

FIGS. 5(a) to (h) show graphs representing distribution of BMD densityin plane for low oxygen concentration wafers.

FIG. 6 shows an explanatory diagram for a method of reducing fluctuationof oxygen precipitation nuclei density according to position in crystal.

FIG. 7 shows an explanatory diagram for a method of increasing oxygenprecipitation nuclei density in NV-region.

FIG. 8 shows a chart representing distribution of various defects, inwhich the horizontal axis indicates position in the crystal along theradial direction and the vertical axis indicates F/G value.

FIG. 9 is a schematic view showing a single crystal pulling apparatusbased on the CZ method used in the present invention.

BEST MODE FOR CARRYING OUT OF THE INVENTION

Hereafter, the present invention will be explained in more detail.

The inventors of the present invention investigated thermal stability ofgrown-in precipitation nuclei by performing the following experiments.

First, several kinds of wafers having different defect regions wereprepared, and subjected to the following heat treatments.

After the wafers were loaded into a furnace set at T° C. (T=700, 800,900, 1000), the temperature was increased at a rate of 1.5° C./minutefrom T° C. to 1050° C., and maintained at 1050° C. for 4 hours. In thisheat treatment, grown-in precipitation nuclei stable at a temperature ofT° C. or higher were grown to have such a size that they should notdisappear at 1050° C. by the temperature increase at a slow rate andthey were further grown to have a size detectable by a conventionalevaluation method by maintaining them at 1050° C. for 4 hours.

An important point was that the grown-in precipitation nuclei were fullygrown by the optimization of temperature increasing rate, and theconditions were selected so that precipitation nuclei should not benewly generated in the heat treatment step. Therefore, the oxideprecipitate (BMD: Bulk Micro Defect) density after this heat treatmentwould represent a density of grown-in precipitation nuclei stable at atemperature of T° C. or higher. The BMD density after the heat treatmentwas measured by infrared laser scattering tomography method (LST). Themeasurement were performed at position at a distance of 10 mm from theedge and those at distances increased by a 10 mm interval toward thecenter for a region having a depth of about 50-180 μm from the surface.

As a result of the experiments described above, it was found that thethermal stability of grown-in precipitation nuclei was influenced by thedefect regions defined by the OSF ring (region inside the ring, ringregion, and region outside the ring), oxygen concentration, and theposition along the crystal axis direction. The results will be explainedbelow.

(1) Relationship Between Grown-in Precipitation Nuclei and DefectRegions

The relationship between the heat treatment starting temperature T° C.and BMD density is shown in FIG. 1. The closed symbols represent theresults for low oxygen concentration wafers (12-14 ppma) and the opensymbols represent the results for high oxygen concentration wafers(15-17 ppma). While the different shapes of the symbols indicate thedifferent wafers (different pulling conditions), such difference willnot be discussed herein.

In these graphs, for example, the BMD density of 1×10⁹/cm³ for 700° C.indicates that the density of grown-in precipitation nuclei that canremain at 700° C. is 1×10⁹/cm³. In theory, if the temperature becomeshigher, size of the precipitation nuclei that can remain at thattemperature (critical size) will become larger. Large precipitationnuclei that can remain at a high temperature can remain also at a lowtemperature. Therefore, BMD density for 700° C. means a density of allprecipitation nuclei that can remain at a temperature of 700° C. orhigher.

(1-1) Region Inside OSF Ring (V-rich Region)

FIG. 1(a) shows the results for the region inside the OSF ring. As theheat treatment starting temperature became higher, the BMD densitybecame lower. That is, a larger size of precipitation nuclei provided alower density of them. In particular, above 900° C., the density becameextremely low, i.e., less than the order of 10⁶/cm³. This shows that thedensity of thermally stable comparatively large grown-in precipitationnuclei is extremely low in the region inside the OSF ring. Further,since they show strong temperature dependency, it is expected thatdifferent device production process conditions (initial stage heattreatment temperatures) would provide significantly different BMDdensities.

(1-2) OSF Ring Region and NV-region

The results in the OSF ring region and the NV-region are shown in FIGS.1(b) and (c), respectively. The results for the both showedsubstantially the same tendency. It can be seen that the BMD density for900° C. or higher was clearly higher than the region inside the OSFring. That is, the density of thermally stable precipitation nuclei washigh. It can be seen that, since the temperature dependency became weak,difference of the device production process conditions did not providesignificant change of BMD density. The OSF ring region and the NV-regionshowed significant difference, i.e., OSFs were generated or not byoxidation at high temperature. It is considered that this differenceoriginated in the difference in density of precipitation nuclei stableat a temperature higher than 1000° C.

(1-3) NI- and I-rich Regions

The results in the NI- and I-rich regions are shown in FIG. 1(d).Although there are few data, the tendency was substantially the same asthe region inside the OSF.

From the above results, it was found that, by measuring BMD densities inheat treatments of which starting temperature were 800° C. and 1000° C.,for example, the kind of defect region of a corresponding wafer could bedetermined.

(2) Oxygen Concentration Dependency

(2-1) Case in which Influence of Position in Crystal is Taken IntoConsideration

The oxygen concentration dependency of BMD density is shown in FIG. 2.The different symbols (circles, triangles and squares) representdifferent positions in crystal, and classified into those at distancesfrom crystal shoulder of 0-40 cm, 40-80 cm and 80 cm or more.

FIG. 2(a) shows difference of BMD density at 700° C. and BMD density at800° C. This difference indicates density of only grown-in precipitationnuclei that can remain at 700° C., but cannot remain at 800° C., i.e.,extremely small precipitation nuclei. These results show that the oxygenconcentration dependency of the density of small precipitation nucleiwas strong and the density decreased when the oxygen concentrationbecame low. Further, it can be also seen that the density also showeddependency for the position in crystal, and the density decreased atpositions at distances of 80 cm or more from the crystal shoulder.

Although the oxygen concentration dependency was also seen for thedensity of precipitation nuclei stable at 800-900° C. as shown in FIG.2(b), the influence of the position in crystal was not clearly observed.

On the other hand, as shown in FIGS. 2(c) and (d), it can be seen thatthe density of only large precipitation nuclei stable at 900-1000° C. orabove 1000° C. showed substantially no oxygen concentration dependencyand no dependency for the position in crystal.

(2-2) Case in which Influence of Defect Region is Taken intoConsideration

The results for a case in which the influence of defect region was takeninto consideration for the oxygen concentration dependency of BMDdensity are shown in FIG. 3. The data for positions near boardersbetween defect regions are omitted. As the temperature region shifted toa higher region, i.e., precipitation nucleus size became larger, theinfluence of defect regions was more clearly observed. As shown in FIGS.3(c) and (d), the density of precipitation nuclei stable at 900-1000° C.or above 1000° C. was clearly higher in the OSF ring region and theNV-region. However, there was almost no oxygen concentration dependency.

Combining the results of (2-1) and (2-2), the following can beappreciated.

Although density of relatively small grown-in precipitation nucleistrongly depends on the oxygen concentration and position in crystal, itis unlikely to be influenced by the defect regions. On the other hand,density of large grown-in precipitation nuclei stable at hightemperature is hardly influenced by the oxygen concentration or positionin crystal, but it is strongly dependent on the defect regions.

(3) Uniformity of Oxygen Precipitation in Plane

As described above, it was found that the thermal stability of grown-inprecipitation nuclei was strongly dependent on the defect regionsdefined based on the OSF ring. Therefore, it is readily expected that awafer having two or more defect regions shows bad uniformity of oxygenprecipitation in its plane. The results showing it are shown in FIGS.4(a) to 4(f) and 5(a) to 5(h).

(3-1) Case of High Oxygen Concentration Wafers

The results for high oxygen concentration wafers (15-17 ppma) are shownin FIGS. 4(a) to 4(f). The different symbols represent different heattreatment starting temperatures. Wafers containing two or more kinds ofdefect regions showed bad uniformity of BMD density in plane when thetemperature was high (FIGS. 4(c) to (f)). This is because the density oflarge grown-in precipitation nuclei stable at a high temperature isstrongly influenced by the defect regions as described in (2). However,if the temperature became low, the uniformity in plane was improved.This is because the density of small grown-in precipitation nuclei ishardly influenced by the defect regions, but it is strongly dependent onthe oxygen concentration. As for the influence of the device productionprocess, it is estimated from these results that uniformity of the BMDdensity in plane would not be degraded if a low temperature processusing a low first stage temperature (700-800° C.) is used, whereas theuniformity in plane would be degraded if a high temperature processusing a high first stage temperature (about 900° C.) is used. This isconsidered to constitute a problem for conventional low defect crystals.

(3-2) Case of Low Oxygen Concentration Wafers

The results for low oxygen concentration wafers (12-14 ppma) are shownin FIGS. 5(a) to 5(h). Compared with the high oxygen concentrationwafers, the distribution of BMD density in plane was degraded in the lowoxygen concentration wafers even when the heat treatment startingtemperature was low. This is because the density of small precipitationnuclei decreases when the oxygen concentration is low, and hence largeprecipitation nuclei strongly influenced by the defect regions becomedominant at any temperature. From these results, it is suggested thatlow oxygen concentration wafers would show bad uniformity of BMD densityin plane in any kind of device production process.

The inventors of the present invention further studied assiduously basedon the findings obtained in the above (1) to (3), and made the followingconsiderations as for methods for stably obtaining the BMD density byany type of device production processes covering from a high temperatureprocess to a low temperature process. Thus, they accomplished thepresent invention. A conceptual diagram concerning the followingdiscussions is shown in FIG. 6.

<Consideration 1>

Method for Reducing Fluctuation of BMD Depending on Position in Crystal

The substantial problem in the control of oxygen precipitation is itssignificant fluctuation depending on position in crystal. In theexperiments performed by the inventors of the present invention, it wasfound that the influence of position in crystal was significant for thedensity of small precipitation nuclei stable at 700-800° C. It isconsidered that these precipitation nuclei are formed in a temperaturerange of 700° C. or lower in the thermal history of crystal. That is,the fluctuation depending on the position in crystal may be reduced byusing the same thermal history at 700° C. or lower for the top portion(shoulder side (K side)) and the bottom portion (tail side (P side)) ofa crystal. However, this is extremely difficult. Then, it was consideredthat the fluctuation depending on the position in crystal could bereduced by lowering the density. Based on the results shown in FIG. 2,it is considered that the oxygen concentration must be 14 ppma or lowerin order to lower the density of small precipitation nuclei. If itexceeds 14 ppma, it becomes difficult to decrease the fluctuationdepending on the position in crystal, which is the object of the presentinvention. This conception of eliminating the dependency of oxygenprecipitation on the position in crystal can be also applied to a caseusing nitrogen doping, and the oxygen concentration can be similarlymade to be 14 ppma or less.

<Consideration 2>

Method for Forming Thermally Stable Grown-in Precipitation Nuclei

In order to stably obtain BMDs for different device productionprocesses, thermally stable large precipitation nuclei are required. Thedensity of large precipitation nuclei strongly depends on the type ofdefect region, and the density becomes high in the OSF ring region andthe NV-region. However, since OSFs may be generated during a hightemperature process in the OSF ring region, it is thought that theoptimum region is the NV-region.

Taking Consideration 1 into account, it can be said that a wafer thatstably provides BMDs regardless of position in crystal and deviceproduction process should be a wafer having an oxygen concentration of14 ppma or less and an NV-region for the whole plane (or NV-regioncontaining an OSF ring region for the whole plane or the OSF ring regionfor the whole plane). However, in the conventional NV-region, theprecipitation nuclei density is in an order of 10⁷/cm³, and it cannotnecessarily be said that it is sufficient.

<Consideration 3>

Method for Increasing Precipitation Nuclei Density in NV-region

Since the density of thermally stable large precipitation nuclei hardlydepends on the oxygen concentration as shown in FIG. 3, increase of thedensity cannot be expected by using a higher oxygen concentration.

Now, the mechanism of the formation of stable precipitation nuclei inthe NV-region is considered. A conceptual diagram therefor is shown inFIG. 7. In NV-region, by controlling the crystal pulling condition: F/G(F: pulling rate, G: temperature gradient near growth surface),supersaturation degree of vacancies is decreased and therefore theformation of voids is inhibited. This makes the vacancies more excessivein the NV-region compared with a region in which voids are formed at atemperature lower than the void formation temperature range. Thephenomenon that the oxygen precipitation nucleation is accelerated byexcessive vacancies at a relatively high temperature has been confirmedby various experiments. That is, it is considered that, in theNV-region, the precipitation nucleation is accelerated by excessivevacancies at a relatively high temperature (considered to be in therange of about 1000-750° C.). When the nuclei are formed at a hightemperature, they can be sufficiently grown by the subsequent coolingstep, and therefore they become thermally stable precipitation nuclei oflarge size.

Based on the mechanism described above, the density of thermally stableprecipitation nuclei will be increased when the vacancy concentrationbecomes higher. However, when the vacancy concentration becomes higher,the void formation is promoted, and as a result, the concentration ofvacancies contributing to the precipitation nucleation is decreased.Therefore, there is required a certain method that can inhibitaggregation of the vacancies even if the vacancy concentration is high.

Then, use of nitrogen doping was conceived. While it is shown in FIG. 6,if nitrogen is doped, aggregation of the excessive vacancies will beinhibited and remaining excessive vacancies will promote theprecipitation nucleation at a high temperature. As a result, the densityof thermally stable precipitation nuclei will increase. However, theregion inside the OSF ring cannot be used, because many micro COPs(micro void defects) present in that region degrade devicecharacteristics. Therefore, an NV-region of a nitrogen-doped crystal ispreferred, and it shows an effect that stable oxide precipitates can beobtained without depending on the device production process.

An exemplary structure of apparatus for pulling a single crystal by theCZ method used in the present invention will be explained hereafter byreferring to FIG. 9. As shown in FIG. 9, the apparatus 30 for pulling asingle crystal is constituted by a pulling chamber 31, crucible 32provided in the pulling chamber 31, heater 34 disposed around thecrucible 32, crucible-holding shaft 33 for rotating the crucible 32 androtation mechanism therefor (not shown in the figure), seed chuck 6 forholding a silicon seed crystal 5, wire 7 for pulling the seed chuck 6,and winding mechanism (not shown in the figure) for rotating and windingthe wire 7. The crucible 32 is composed of an inner quartz crucible foraccommodating a silicon melt (molten metal) 2, and an outer graphitecrucible. Further, a heat insulating material 35 surrounds the outsideof the heater 34.

Further, in order to establish production conditions used for theproduction methods of the present invention, an annular solid-liquidinterface heat insulating material 8 is provided around the periphery ofthe solid-liquid interface 4 of the crystal 1, and an upper surroundingheat insulating material 9 is provided thereon. This solid-liquidinterface heat insulating material 8 is provided so as to form a gap 10of 3-5 cm between its lower end and the melt surface 3 of silicon melt2. The upper surrounding heat insulating material 9 may not be useddepending on the conditions. Further, a cylindrical cooling apparatusnot shown in the figure may be provided for cooling the single crystalby blowing cooling gas or shielding the radiant heat.

As another method, the so-called MCZ method is recently often used, inwhich a non-illustrated magnet is additionally installed outside thepulling chamber 31 in the horizontal direction, and a horizontal orvertical magnetic field is applied on the silicon melt 2 so as toprevent convection of the melt and realize stable growth of a singlecrystal.

As an exemplary method for pulling a silicon single crystal by using theaforementioned apparatus 30, a method for growing a nitrogen-dopedsingle crystal will be explained hereafter. First, a silicon polycrystalraw material of high purity is melted in the crucible 32 by heating itto a temperature higher than the melting point (about 1420° C.). At thispoint, silicon wafers having a nitride film, for example, are added inorder to dope nitrogen. Then, a tip end of the seed crystal 5 is broughtinto contact with the surface of the melt 2, or immersed into the melt 2at its approximate center portion by reeling out the wire 7.Subsequently, the crucible-holding shaft 33 is rotated in an optionaldirection, and at the same time the seed crystal 5 is pulled upwardly bywinding up the wire 7 with rotating the wire to start the growing ofsingle crystal. Thereafter, a single crystal ingot 1 approximately in acylindrical shape can be obtained by suitably controlling the pullingrate and temperature.

In this case, according to the present invention, in order to controlthe temperature gradient in the crystal, the gap 10 between the lowerend of the solid-liquid interface heat insulating material 8 and themelt surface 3 of the silicon melt 2 is controlled, at the same time,the solid-liquid interface heat insulating material 8 in a ring shape isdisposed in a region in which the temperature of the crystal near themelt surface is in a temperature range of, for example, 1420-1400° C.,in the space surrounding a liquid portion of the single crystal ingot 1above the melt surface in the pulling chamber 31, and the uppersurrounding heat insulating material 9 is disposed thereon. Further, ifrequired, the temperature can be controlled by providing an apparatusfor cooling the crystal above the insulating material so that thecrystal can be cooled by blowing a cooling gas to it from the above, andproviding a collar for shielding the radiant heat at the lower part of acylinder.

Hereafter, the present invention will be explained with reference to thefollowing specific examples. However, the present invention is notlimited to these.

EXAMPLE 1

Raw material polycrystal silicon was charged into a quartz cruciblehaving a diameter of 24 inches, and a single crystal ingot of p-typehaving a diameter of 8 inches, orientation of <100> and interstitialoxygen concentration of 12-14 ppma (according to JEIDA (Japan ElectronicIndustry Development Association) standard) was pulled by the CZ method,while F/G was controlled so that a single crystal ingot having NV-regionfor the whole plane should be formed. In this pulling operation, theoxygen concentration was controlled by controlling rotation of thecrucible during the pulling. Two kinds of single crystal ingots werepulled as described above with or without adding silicon wafers having apredetermined amount of silicon nitride films to the polycrystal rawmaterial beforehand, and mirror-polished wafers having NV-region for thewhole plane (nitrogen-doped wafer and non-nitrogen-doped wafer) wereproduced from the above single crystal ingots.

Each nitrogen-doped wafer was sliced at a position having a nitrogenconcentration of 1×10¹⁴ number/cm³, which was calculated from the amountof nitrogen added to the raw material polycrystal and the segregationcoefficient of nitrogen, and processed into the wafer.

These wafers were subjected to a heat treatment, in which the waferswere loaded into a furnace set at 1000° C. instead of a first stage heattreatment of device production process, and then the temperature wasincreased from 1000° C. to 1050° C. at a rate of 1.5° C./min andmaintained at 1050° C. for 4 hours. Then, BMD density of the wafersafter the heat treatment was measured by the infrared laser scatteringtomography method (LST). The measurement was performed at a position ata distance of 10 mm from the edge and those at distances increased by a10 mm interval toward the center for a region having a depth of about50-180 μm from the surface.

As a result, the BMD density was 3×10⁹ to 8×10⁹ number/cm³ for thenitrogen-doped wafer, and 2×10⁷ to 5×10⁷ number/cm³ for thenon-nitrogen-doped wafer. Therefore, it was found that both of thewafers could have a BMD density of significant level with superioruniformity in plane even though they were subjected to a heat treatmentat a relatively high temperature for the first stage of the deviceproduction process. That is, this indicates that grown-in oxygenprecipitation nuclei of large size stable at a high temperature wereuniformly formed in plane for both of the wafers. Further, in the caseof the nitrogen-doped wafer, a quite high BMD density was obtained andit was found to have extremely high gettering effect.

EXAMPLE 2

A wafer of which defect region was unknown was divided into two pieces.One of the pieces was loaded into a furnace set at 800° C., and thetemperature was increased at a rate of 1.5° C./min from 800° C. to 1050°C. and maintained at 1050° C. for 4 hours so that precipitation nucleishould grow to have a detectable size.

Similarly, the other piece was loaded into a furnace set at 1000° C.,and the temperature was increased at a rate of 1.5° C./min from 1000° C.to 1050° C. and maintained at 1050° C. for 4 hours so that precipitationnuclei should grow to have a detectable size.

Then, BMD density of each piece after the heat treatment was measured byinfrared laser scattering tomography method (LST). The measurement wasperformed at a position at a distance of 10 mm from the edge and thoseat distances increased by a 10 mm interval toward the center for aregion having a depth of about 50-180 μm from the surface.

The BMD density was 1×10⁹ number/cm³ for the wafer for which thetemperature was increased from 800° C., and 3×10⁶ number/cm³ for thewafer for which the temperature was increased from 1000° C. That is, thedifference of BMD densities for the temperatures of 800° C. and 1000° C.was in an order of two figures or more, and therefore it could bedetermined that the defect region of this wafer was a V-rich regioninside the OSF ring.

The present invention is not limited to the embodiments described above.The above-described embodiments are mere examples, and those having thesubstantially same structure as that described in the appended claimsand providing the similar functions and advantages are included in thescope of the present invention.

For example, the aforementioned embodiments were explained for caseswhere silicon single crystals having a diameter of 8 inches were grown.However, the present invention is not limited to them, and can also beapplied to silicon single crystals having a diameter of 6 inches orless, or 10-16 inches or larger.

What is claimed is:
 1. A silicon wafer having an NV-region or anNV-region containing an OSF ring region for the entire plane of thesilicon wafer and having an interstitial oxygen concentration of 14 ppmaor less.
 2. A silicon wafer obtained by slicing a silicon single crystalingot grown by the Czochralski method with nitrogen doping, wherein thesilicon wafer has an NV-region or an NV-region containing an OSF ringregion for its entire plane.
 3. The silicon wafer according to claim 2,wherein nitrogen concentration doped in the silicon wafer is 1×10¹⁰ to5×10¹⁵ number/cm³.
 4. The silicon wafer according to claim 3, whereininterstitial oxygen concentration of the silicon wafer is 14 ppma orless.
 5. The silicon wafer according to claim 2, wherein interstitialoxygen concentration of the silicon wafer is 14 ppma or less.
 6. Amethod for producing a silicon wafer, wherein, when a silicon singlecrystal is grown by the Czochralski method, the crystal is pulled withsuch conditions as present in an NV region or an OSF ring region in adefect distribution chart showing defect distribution which is plottedwith D [mm] in the horizontal axis and F/G [mm²/° C.·min] in thevertical axis, wherein D represents a distance between center of thecrystal and periphery of the crystal, F [mm/min] represents a pullingrate and G [° C./mm] represents an average temperature gradient in thecrystal along the crystal pulling axis direction in the temperaturerange of from the melting point of silicon to 1400° C., so thatinterstitial oxygen concentration should become 14 ppma or less.
 7. Amethod for producing a silicon wafer, wherein, when a silicon singlecrystal is grown by the Czochralski method, the crystal is pulled withsuch conditions as present in an NV region or an OSF ring region in adefect distribution chart showing defect distribution which is plottedwith D [mm] in the horizontal axis and F/G [mm²/° C.·min] in thevertical axis, wherein D represents a distance between center of thecrystal and periphery of the crystal, F [mm/min] represents a pullingrate and G [° C./mm] represents an average temperature gradient in thecrystal along the crystal pulling axis direction in the temperaturerange of from the melting point of silicon to 1400° C., and withnitrogen doping.
 8. The method for producing a silicon wafer accordingto claim 6, wherein nitrogen concentration to be doped is 1×10¹⁰ to5×10¹⁵ number/cm³.
 9. The method for producing a silicon wafer accordingto claim 8, wherein, when a crystal is grown by the CZ method, thecrystal is pulled so that interstitial oxygen concentration shouldbecome 14 ppma or less.
 10. The method for producing a silicon waferaccording to claim 7, wherein, when a crystal is grown by the CZ method,the crystal is pulled so that interstitial oxygen concentration shouldbecome 14 ppma or less.
 11. A method for evaluating defect regions of asilicon wafer produced by the CZ method, wherein a defect region of asilicon wafer to be evaluated is evaluated by comparing at least two ofoxide precipitate densities measured by the following steps: (1) a waferto be evaluated is divided into two or more pieces (A, B, . . . ), (2)Wafer piece A among the divided pieces is loaded into a heat treatmentfurnace maintained at a temperature of T1 [° C.] selected from atemperature range of 600-900° C., (3) the temperature is increased fromT1 [° C.] to a temperature of 1000° C. or higher, T2 [° C.], at atemperature increasing rate of t [° C./min] (provided that t is 3°C./min or less), and the temperature is maintained until oxideprecipitates in Wafer piece A grow to have a detectable size, (4) Waferpiece A is unloaded from the heat treatment furnace, and oxideprecipitates density in the wafer piece is measured, (5) another waferpiece among the divided wafer pieces, Wafer piece B, is loaded into aheat treatment furnace maintained at a temperature of T3 [° C.] selectedfrom a temperature range of 800-1100° C. (provided that T1<T3<T2), (6)the temperature is increased from T3 [° C.] to the temperature of T2 [°C.] at the temperature increasing rate of t [° C./min], and thetemperature is maintained until oxide precipitates in Wafer piece B growto have a detectable size, and (7) Wafer piece B is unloaded from theheat treatment furnace, and oxide precipitates density in the waferpiece is measured.